Interpretive Summary: It is well established that reproductive function is metabolically gated. Inadequate nutrition is characterized by low blood levels of pituitary hormones, such as luteinizing hormone (LH), that are necessary for stimulation of the reproductive system. A lack of these hormones results in delayed puberty, irregular heat cycles and failure of animals to breed. The recently discovered protein, leptin, secreted by fat cells in response to changes in body weight or energy, stimulates metabolism, LH secretion and in domestic farm animals. Leptin may serve as a signal linking the body's energy regulating system with the reproductive system. Literature is reviewed identifying specific action of leptin in regulating reproduction in domestic farm animals.

Technical Abstract:
It is well established that reproductive function is metabolically gated. However, the mechanisms whereby energy stores and metabolic cues influence appetite, energy homeostasis and fertility are yet to be completely understood. Adipose tissue is no longer considered as only a depot to store excess energy. Recent fining have identified numerous genes several neurotrophic factors, interleukins, insulin like growth factor binding protein (IGFBP-5), ciliary neurotrophic factor, (CNTF) and NPY are expressed by adipose tissue during puberal development. These studies demonstrate for the first time the expression of several major adipokines or cytokines in pig adipose tissue which may influence local and central metabolism and growth. Leptin appears to be the primary metabolic signal and is part of the adipose tissue-hypothalamic regulatory loop in the control of appetite, energy homeostasis and luteinizing hormone (LH) secretion. Leptin’ actions on appetite regulation are mediated inhibition of hypothalamic neuropeptide Y(NPY), stimulation of proopiomelanocortin (POMC) and its effects on gonadotropin releasing hormone (GnRH)/LH secretion are mediated by NPY and kisspeptin (KiSS) mediating leptins’ effects on gonadotropin releasing hormone (GnRH) release. Thus, leptin appears to an important link between metabolic status, neuroendocrine axis and subsequent fertility in the gilt and sow.